The invention generally relates to a method for manufacturing freestanding films of material having a thickness less than one-half micron and even as thin as one-tenth micron. The method has been found to have particular utility in the preparation of films of superconducting YBa.sub.2 Cu.sub.3 O.sub.x, a material which has received considerable attention due to its promising electrical and mechanical properties. This method also has utility in processing a variety of metals, ceramics, oxides or other materials which can be deposited as films.
Superconducting films are used in a variety of electronic and microwave devices. One application of particular interest involves highly sensitive resistance thermometers, that is, bolometers, for use in the study of infrared spectra. Freestanding films of superconducting material are much more desirable for use as bolometer elements than are superconducting films which are integral with substrates. With bolometers, the receipt of infrared radiation by the superconducting element causes a change in the overall resistance of the element. Temperature changes and their value can be detected by passing a constant current across the element and monitoring changes in voltage.
Composites of superconducting films deposited onto substrates are currently available as bolometers. The heat capacity of the substrate material contributes to the overall heat capacity of the bolometer. The heat capacity of substrate material typically may be, for example, on the order of forty times the heat capacity of the superconducting material. A significant amount of radiation is therefore absorbed by the substrate rather than by the superconducting film. The sensitivity of the bolometer is sacrificed because a significant quantity of heat variation is absorbed by the substrate, not by the film, and thus these changes in radiation are not translated into detectable changes in voltage across the superconducting element. This adverse heat-absorptive contribution of the supporting substrate can be eliminated through the use of films of superconducting material which are freestanding, that is, not integral with supporting substrates. The lower thermal mass of a freestanding film, as opposed to a substrate/thin film composite, has been found to result in an approximately tenfold increase in bolometer sensitivity. Freestanding material processed according to this invention therefore allows for the detection of much smaller fluctuations in infrared radiation.
In addition to being freestanding, superconducting films used as bolometers are desirably as thin as practically possible. For example, a bolometer which is 50 microns thick will have a heat capacity, or thermal mass, 100 times greater than a bolometer of the same area which is only one-half micron thick. As discussed above, a lower thermal mass corresponds to greater bolometer sensitivity.
Another application of particular utility for thin freestanding films of a particular material is the measurement of the optical properties of the material. Reflectance, rather than transmission, is commonly measured for thin films deposited onto substrates which are opaque in the spectral region of interest. The availability of material as thin freestanding films rather than only as films on substrates facilitates the measurement of transmission. One instance where transmission measurements are of particular interest involves the determination of the energy gap of superconducting materials. The energy gap is the range of electromagnetic energy in which the material has zero absorption, when applied to material in the superconducting state. Although present technical limitations do not permit determination of the energy gap directly with much precision, it can be determined indirectly by optical measurements as set forth in Mattis and Bardeen, Theory of Anomalous Skin Effect in Normal and Superconducting Metals, Phys. Rev. 111 (Jul. 15, 1958). Transmission measurements, as contrasted with reflectance measurements, generally give a more precise value of energy gap using the Mattis-Bardeen theory.
Thin freestanding films of ceramic, metal or oxide are similarly advantageous to other technologies where the substrate is an annoyance or an obstacle or otherwise interferes with optimal operation or utility.
Thin films have been deposited onto substrates by various methods including laser ablation, sputtering, backscattering, electron beam deposition, molecular beam epitaxy, spray pyrolysis and the like. Superconducting material such as YBa.sub.2 Cu.sub.3 O.sub.x has been deposited to form thin films on various substrates including SrTiO.sub.3, KTiO.sub.3, MgO and ZrO.sub.2.
A variety of methods have been used to prepare freestanding thin films from the substrate/thin film composites resulting from the deposition methods described above. One method available heretofore for removing the substrate from the film is to preferentially dissolve the substrate into an appropriate solvent as described generally in S. H. Maxman, Target Preparation Techniques, Nuclear Instr. and Meth. 50, p.56 (1967). This deposition/dissolution process is inappropriate for the manufacture of films of certain materials due to the potential incompatibility of the solvent and the material. Contamination of the film material by the solvent and its by-products can also be a problem. For example, when producing films of YBa.sub.2 Cu.sub.3 O.sub.x, the presence of water in the solvent undesirably results in the formation of Ba(OH).sub.2 in the film and, upon further reaction, BaCO.sub.3.
Another method heretofore available for separating a film from its substrate and thus obtaining freestanding films is disclosed in D. S. Ginley et al., Freestanding Thick Films of YBa.sub.2 Cu.sub.3 O.sub.6,9 by Screenprinting Techniques, J. Mater. Res., vol. 4, No. 3 (1989). According to this method, sintered compacts of YBa.sub.2 Cu.sub.3 O.sub.6.9 were milled into powder form. Dry ground YBa.sub.2 Cu.sub.3 O.sub.6.9 powder and powder/alcohol mixtures were directly screenprinted onto silica substrates. The composite comprising the screenprinted film and substrate was sintered to effectuate resintering of the YBa.sub.2 Cu.sub.3 O.sub.6.9. Freestanding material was obtained because, upon cooling, an interfacial reaction between the substrate and the sintered film resulted in debonding of the film from the substrate. The minimum thickness of films obtainable by this method is limited by the mechanical strength of the material and by the existence of a substantial reaction zone within the ceramic material. This reaction zone creates a mechanically weak interface vulnerable to shearing caused by mismatch in thermal expansion between the substrate and the ceramic material. The results obtained with sheets on the order of 20 microns thick were erratic. Sheets of less than 50 microns did not have the mechanical strength to debond from large areas of substrates. Additionally, the elemental distribution within the final product was determined to be nonhomogeneous.
Freestanding thin films have also been produced by stripping a thin mass of material from a bulk mass with adhesive tape or by physically cleaving a thin layer from a bulk sample. Disadvantageously, however, the thickness, uniformity and surface area of films produced by such techniques are difficult to control. The reliability and repeatability of these methods are therefore highly sensitive to the degree of skill and attention exercised by the operator.
There is, therefore, a need for a reliable and repeatable method of producing freestanding thin films on the order of less than a half micron in thickness of ceramics, oxides, metals and other materials which may be deposited as thin films.